1. Field of the Invention
The invention relates to the purification of crystallizable chemicals.
2. Description of the Prior Art
A process of centrifugal crystallization is disclosed in U.S. Pat. No. 3,844,725.
Existing crystallization systems work with a number of steps such as:
crystallization by solvent evaporation and/or cooling, PA1 separation of obtained crystals from mother liquor, PA1 removal of impurities occluded on the crystal surface by washing with solvent (or product melt in some cases), PA1 separation of the crystals from the wash, PA1 removal of the remnants of the solvent from the washed product by drying, PA1 melting the crystals (in some cases).
Most of the enumerated steps are carried out separately, a few systems combine two or more steps; none is capable of executing the entire procedure in a single operation and in a single apparatus.
In the crystallization of organics, the most effective method for removing impurities from the crystal surface appears to be the counter-current washing with their own melt. Most of the present systems work with a tightly packed bed of crystals where proper washing is hindered by the presence of contact points and contact facets. Some others achieve the counter-current washing in a number of reslurrying and separating stages, or in a vertical column employing internal screw-type conveying means.
Most of the required equipment is heavy and cumbersome, demanding on manpower, maintenance, energies, instrumentation etc.; operational difficulties are encountered with handling crystal slurries and freezups. All these facts are so discouraging that in the selection of a purification process in many a case distillation is chosen as a resort for some crystallizable compounds, in spite of its high energy demand and, in some cases, in spite of the necessity of first chemically converting impurities with close boiling points.